Add guardrails for cbepctl mem_low_wat and mem_high_wat

We recently had an unfortunate incident where we ran something like this on a prod cluster while attempting to slightly adjust the high watermark down to 80%:

cbepctl localhost:11210 -u $USER -p $PASS -b $BUCKET set flush_param mem_high_wat 80  

This set the high watermark to 80 bytes, not 80% of ram quota, with unintended and dire consequences.

In the name of safety and operator-friendliness I would suggest that cbepctl would either refuse bare numeric values less 100 bytes or else coerce them into percentages for these watermark settings. I haven't yet been able to think of a legitimate reason where you'd want to set the low or high watermark to 80 bytes.